Precipitation hardening steel

ABSTRACT

A low carbon, precipitation hardening steel characterized by good room temperature and good high temperature properties, said steel containing as essential elements about 0.20 percent carbon, 1.50 percent chromium, 1.75 percent nickel, 2.00 percent molybdenum, 2.65 percent copper, and the balance substantially iron with residual elements in normal amounts.

1 United States Patent 1151 3,661,565

Harvey 1 1 May 9, 1972 54] PRECIPITATION HARDENING STEEL 1,987,8411/1935 Rittershausen 148/36 x 2,012,765 8/1935 Marthourey ..148/31 [72]Invent 2,327,490. 8/1943 Bagsar ..75/128R [73] Assignee: Metaltronics,lnc., Orchard Lake, Mich.

FOREIGN PATENTS OR APPLICATIONS [22] Filed: Aug. 4, 1969 387,865 2/1933Great Britain ..75/125 1 1 pp 855,065 405,643 1/1934 Great Britain..148/142 500,200 2/1939 Great Britain ..75/125 Dam 688,289 2 1940Germany ..75 125 [63] Continuation-impart of Ser. No. 559,679, June 20,

1966, abandoned. Primary Eranii11er-Charles N. Lovell [52] US. Cl..75/125, 75/1285, 75/1289, [57] ABSTRACT 148/31 148/36 A low carbonpreci itation hardening steel characterized b [51] 1nt.C1 ..C22c 39/20,C22c 41/02 00d room i and 00d hi h tem ermure m [58] Field ofSearch..75/123,125,128, 128.5,128.9; P g g P P P t1es, sa1d steel contammg asessentlal elements about 0.20 per- 148/142,31,36

cent carbon, 1.50 percent chromium, 1.75 percent mckel, [56] ReferencesCited 2.00 percent molybdenum, 2.65 percent copper, and the balancesubstantially 11'01'1 w1th residual elements 1n normal UNITED STATESPATENTS amountsr 1,957,427 5/1934 Bucholtz ..75/125 4Claims, 1 DrawingFigure N 0) Ch O ois a TOTAL FERRlTE Accovms EFF-SGT PRECIPITATIONHARDENING STEEL This application is a continuation in part of mycopending application, Ser. No. 559,679 filed June 20, 1966, nowabandoned, entitled Precipitation Hardening Steels.

The present invention relates to a precipitation hardening compositionwhich posseses excellent room temperature properties as well as goodphysical properties at elevated temperatures. In addition thecomposition of the present invention is characterized by excellentmachinability, high toughness, and minimum distortion on heat treatment.Furthermore the composition of the present invention is designed toprovide for high physical properties at a cost which is a fraction ofthe cost of high strength precipitation hardening alloys presentlyavailable.

In general the low carbon, precipitation hardening compositions offeradvantages in minimized distortion which cannot be duplicated by themore conventional, higher carbon, heat treating grades. Briefly, twosteps are required to heat treat the precipitation hardening steels andthe mechanism of hardening and the results are quite different from thehigher carbon steels which are hardened and tempered.

The precipitation hardening compositions are first given a solutiontreatment which consists in cooling to room temperature from an elevatedtemperature range which is often from about l,800 to about 2,000 F.Following the solution treatment the precipitation hardening steels maybe heated for aging or precipitation hardening which may consist inreheating at lower temperatures up to about 1,100 F. This results inprecipitation hardening.

Many precipitation hardening composition have been attempted in the pastand there are quite a few compositions which are capable of beingprecipitation hardening. As a practical matter, however, the difficultywith most compositions is that the precipitation of the age hardenablephase occurs selectively at the grain boundaries resulting inembrittlement and a marked loss in ductility. Accordingly, thecomposition of the present invention has been directed to theestablishment of a substantially uniform matrix structure, whereinselective precipitation of the hardenable phase at the grain boundariesis substantially avoided.

The most widely used of the precipitation hardening alloys presentlyavailable are the so called Maraging" steels which contain up to 25percent Nickel. However, these steels have limitations, a few of whichare cited as follows:

1. The Maraging steels are very highly alloyed and require vacuummelting for optimum properties. As a result the Maraging steels arequite expensive and sell for about 3.00 per pound.

2. The Maraging steels have relatively poor high temperature propertiesand the hardness and strength fall off sharply at temperatures in excessof about 900 F.

3. Expedients such as refrigeration and cold working between solutionhardening and aging is sometimes applied for maximum properties with theMaraging steels and this represents an item of additional processingcost and inconvenience.

4. In general the Maraging steels have relatively low impact strengthwhich represents a serious deficiency or limitation for which these is acritical need for an improvement.

The deficiencies of the Maraging steels is illustrated by a survey whichappeared in Business Week magazine of Apr. 18, 1964 which reported thateven the most enthusiastic backers of Maraging steels admit some seriousproblems which must be solved before the Maraging steels find wider use.The present invention which is based on a systematic and extensiveinvestigation of many compositions is submitted to correct for certaindeficiencies in the Maraging steels.

The alloys of the present invention combine high strength and ductilityat room and at elevated temperatures to a degree not previouslyobtainable with compositions of the prior art. Also the alloys of thepresent invention are considerably less expensive than the Maragingsteels.

It is therefore an object of the present invention to provideprecipitation hardenable steels which are characterized by excellentphysical properties at room temperature at a cost which is considerablyless than the high strength precipitation hardenable steels presentlyavailable.

Another object of this invention is to provide precipitation hardenablesteels which are characterized by good elevated temperature properties,particularly in the temperature range above about 900 F up to aboutl,l00 F.

Other related objects of this invention include the provision ofprecipitation hardenable steels which are characterized by excellentmachinability, minimized distortion on age hardening, and high ductilityvalues.

Other objects of this invention will be apparent from the specificationand examples.

I have discovered that the foregoing objects can be achieved with asteel of the following approximate composition, considering only theessential alloying elements:

Carbon 0.20

Chromium l.50

Nickel 1.75

Molybdenum 2.00

Copper 2.65

Iron Balance with normal residual elements.

As discussed more fully hereinafter, about 0.05 percent Nitrogen hasbeen found to be beneficial to the machining properties and this may beadded as nitrided ferrochrome. However Nitrogen is not essential toobtain the beneficial properties in accordance with the teachings ofthis invention.

Aluminum also contributes to precipitation hardening and may be added upto about 1.00 percent without serious adverse effects.

The balance is substantially iron with such elements as manganese,silicon, sulphur, and phosphorous in nonnal residual amounts.

The composition of the present invention may extend over a relativelywide range and retain its desirable characteristics for room temperatureand elevated temperature properties. Broadly the composition may fallwithin the following limits of alloying elements:

Carbon About 0.10 to 0.25

Silicon 1.00 maximum Manganese 1.00 maximum Chromium 0.75 to 5.00Molybdenum 1.00 to 4.00

Copper 1.25 to 3.50

Nickel 1.00 to 4.00

Aluminum 1.00 maximum Nitrogen 0.30 maximum The balance is substantiallyiron with residual elements in normal amounts.

The optimum solution treating temperature is from about 1,800' F toabout 1,900 F followed preferably by air cooling to room temperature. Avery satisfactory solution treatment consists in air cooling from theforging, pressing, or rolling operation without annealing. Needless tosay this represents a savings in operating cost. In the solution treatedcondition, alloys of the present invention are about 38 to 43 Rockwell Cin which condition they are readily machinable.

Aging or precipitation hardening may be accomplished by heating to about950 to l, l 00 F for several hours followed by air cooling to roomtemperature. The foregoing treatment results in a hardness of about 48to 52 Rockwell C as will be discussed more fully hereinafter.

The carbon content influences the hardness obtained on precipitationhardening. The carbon should be less than about 0.25 percent andpreferably it should be in the range of about 0.10 to 0.20 percent.

The molybdenum content may be between 1.00 to 4.00 percent andpreferably it should be between 1.50 to 3.00 percent.

Molybdenum contributes to precipitation hardening. However in amountsover about 3.00 percent the machinability is adversely affected.

Copper also contributes to precipitation hardening and also this elementhas an important beneficial effect on machinability. Copper, however,results in df'ficulties in hot working when present in amounts overabout 3.50 percent and this element should preferably be in the rangefrom about 1.50 to about 3.00 percent.

Nickel improves the forgibility of copper bearing steels and is addedfor this purpose as well as for contributing to precipitation hardening.Nickel may be added in the range of about 1.00 to 4.00 percent andshould preferably be added in the range of about 1.25 to 3.00 percent.To ofi'set the detrimental effect of copper on hot workability, thenickel should preferably be present in amounts equivilent to at leastabout 90 percent of the copper content.

Chromium increases the hardenability or the ability to precipitationharden uniformly in large sections. While Chromium may be added withinrelatively wide limits within the range of about 0.75 to 5.00 percentwith satisfactory results, a narrower range of 1.25 to 3.00 percent issuggested.

Aluminum has been added in small amounts up to about 1.00 percent. Itcontributes to precipitation hardening and facilitates nitriding.However aluminum narrows the pouring temperature range and difficultieswere encountered in teeming high aluminum compositions. Aluminum is notan essential element. 1f added, the aluminum should preferably be lessthan about 0.50 percent.

The addition of 0.05 percent nitrogen, which was added as nitridedferrochrome, results in an improvement in machinability in the solutiontreated condition. Nitrogen also contributes somewhat to precipitationhardening. To prevent bleeding ingots as well as from the standpoint ofcost, the nitrogen should preferably be less than about 0.20 percent.

While a broad composition range for alloys of the present invention hasbeen enumerated, it is advantageous in many instances to limit thecomposition to a narrower range of elements for a more economical and insome respects, a more effective use of the alloying elements. Therefore,a narrower, range of composition limits is as follows:

Carbon about 0.10 to 0.20

Silicon about 0.60 maximum Manganese 0.75 maximum Chromium 1.25 to 3.00

Nickel 1.25 to 3.00 Molybdenum 1.50 to 3.00

Copper 1.50 to 3.00

The balance is substantially iron with other elements in normal residualamounts.

EXAMPLE NO. 1

A heat of steel was made in accordance with the principles of thisinvention to the following analysis, all percentages being by weight:

Carbon 0.20 Silicon 0.22 Manganese 0.49 Chromium 2.76 Nickel 2.54Molybdenum 2.78 Copper 2.22 Nitrogen 0.06

In the solution treated condition after air cooling from the forgingoperation the steel was 42 Rockwell C. On tempering or aging, typicalhardness response is as follows:

TENSILE PROPERTIES Tensile properties in the solution treated and agedcondition are as follows:

5 TABLE 1 Hardness Rock- Elonga- N 0. Condition well 0 Tensile Yieldtion BA 1.. Solution treated... 42 219, 300 134,200 13 29. 4 2 do 42210,700 138, 700 12 23. l 3 Aged 1,050 F..." 51 231,400 190,800 13 30.24 .do 51 233, 650 102, 300 13% 30. l 5. Aged1,150 F 45 104,450 165,35013 35.7 6 .do 45 104, 060 166, 500 13% 34. 7

15 V NOTCH CHARPY IMPACT TABLE II Hardness VNotch Rockwell Charpy No.Treatment C Ft. Lbs. Deflection 1 Solution Treated 44 29 .169 2 SolutionTreated 29 3 Aged 1050 F. 50 22 .034 4 Aged 1050" F. 21 5 Aged 1100* F.46 22 .061 6 Aged 1100 F. 20 7 Aged 1150 F. 44 35 .100 8 Aged ll50 F. 379 Aged 1200* F. 38% 43 .160

10 Aged 1200 F. 42

The deflection was determined by piecing the broken halves of the Charpyspecimen together and determining the deviation from a straight edge.Details of this method is reported in an article by R.F. Harvey and Dr.J.A. Berger entitled Evaluation of 5 percent Chromium Hot Work ToolSteels by Impact and Deflection," Metal Treatment and Drop Forging, May1965.

The foregoing physical properties at room and at elevated temperatureswill be recognized by those skilled in the art as being excellent. Thephysical properties at room temperature are not quite as high as, butare comparable with the more expensive Maraging steels. However theelevated temperature properties and the resistance to tempering of thecomposition of the present invention is definitely superior to theMaraging steels.

The size change on precipitation hardening is quite small and is of theorder or magnitude of several hundredths of a thousandth of an inch perinch of length.

I have found the composition of the present invention to be remarkablyfree from heat checking which is a characteristic weakness of theconventional hot work tool steels. I have heated steel of the presentinvention to 2,000 F followed by water quenching and l have repeatedthis cycle 12 times without cracking. The higher carbon, conventionalhot work steels cannot stand this severe treatment. In this connection,I have found steels of the composition of the present invention to beexcellently suited for such hot work applications as dummy blocks andmandrels in extrusion tooling where conditions of water cooling is oftenencountered. Other hot work applications where resistance to heatchecking is desireable includes forging dies, die casting dies, andextrusion dies.

Alloys of the present invention can be gas nitrided where file hardsurfaces are desired. A two stage Floe process to eliminate theformation of an undesirable, brittle surface layer is recommended. Thecase depth is a function of nitriding conditions as follows:

TABLE III Case 1st Stage 2nd Stage The surface hardness after nitridingwas 15-N 94 which is equivilent to Rockwell C 70.

Precipitation hardening alloys of the present invention are freelymachinable in the solution treated condition as illustrated by thefollowing tabulated results of a facing test made on a 6 inch diameterdisc of the steel of the present invention in the solution treatedcondition at 42 Rockwell C.

TABLE IV Machining tests were made with a 15inch square tool bit of M 2high speed steel with 10 side rake, 8 back rake, 8 side relief, and 8end relief. The nose radius was 0.005 in. No lubricant was used.

RPM Depth of Cut Failure 212 0.050 in. 208 ftJmin. 328 0.010 174 fL/min.328 0.025 160 ftJmin.

In a further machining test, 5/16 inch diameter high speed steel drillsat a speed of 328 rpm was used. A constant load of 52 inch pounds wasapplied and the depth of the drilled hole was measured after drillingfor exactly 60 seconds. The results are tabulated in Table V as follows:5

TABLE V Hardness Depth Alloy Rockwell C Drilled Machinability Present 420.130 100 Invention M81 P 20 30 0.092 10 45 Mold Steel Maraging Steel 400.079 61 l8 7: Nickel EXAMPLE NO. 2

A further confirmatory test was made on another heat of steel made inaccordance with the teachings of this invention to the followingcomposition:

Carbon 0.19

Silicon 0.20

Manganese 0.55

Chromium 2.87

Nickel 2.44

Molybdenum 2.60 0 Copper 2.48 6 Nitrogen 0.04

Aluminum 0.18

Keyhole Charpy tests were made on this composition with the followingresults as tabulated in Table VI. 5

1050 F. 2 hr. 51.4 33.7 0.140

In developing precipitation hardening alloys with ultimate physicalproperties, I have found that it is essential that a balance ismaintained between the austenite forming elements and the ferriteforming elements. The various elements vary in their alloying effect inaccordance with the following factors tabulated in Table VII.

TABLE VII Relative Effect of Alloying Elements Austemite Forming FerriteForming Carbon 35 Chromium l Nitrogen 35 Silicon 5 Nickel l Molybdenum 2Copper 1 Manganese 1 Aluminum 10 TABLE VIII EXAMPLE NO. I

Austenite Forming Elements Ferrite Forming Elements Carbon 0.20X35 7.0Chromium 2.76Xl 2.76 Nitrogen 0.06X35 2.1 Silicon 0.22X5 1.10 Nickel2.54X1 2.54 Molybdenum 2.78X2 5.56 Copper 2.22Xl 2.22 Manganese 0.49 l.49 Total Austenite Total Ferrite Alloying Effect 13.86 Alloying Effect9.91

Total austenite alloying eifect 13.86 40 Total ferrite alloying effect9.91

l have found that the total austenite alloying effect should be in therange of about 8 to 16 and the total ferrite alloying effect should bein the range of about 7 to 14. Also the ratio of the total austenitealloying effect to the total ferrite alloying effect should be in therange of about 1.0 to 1.65. This is a critical ratio which should beheld for optimum results.

The FIGURE represents the extent of the useful area of compositionsaccording to the teachings of this invention. Area abcd representsgraphically this useful area of compositions in terms of the totalaustenite alloying effect the total ferrite alloying effect.

The presently available Maraging steels containing 18 percent nickel areexpensive and sell for about 3.00 per pound or more. Metallurgists andthe metalworking industry recognize the critical need for a low costMaraging type, precipitation hardening steel. The present invention isdesigned to fullfill this urgent need.

It is realized that all of the elements in the composition of thepresent invention have been used previously. However, no one haspreviously made or described a low carbon, precipitation hardening steelof the composition of the present invention. In this connection, it isrealized that others have made steels containing copper, nickel,molybdenum etc. William F. Finkl, for example, in U.S. Pat. No.2,104,980 describes an alloy die block steel having the followinganalysis taken from the example of his invention:

The Finkl analysis differs considerably from the composition of thepresent invention. It will be noted also that the I Finkl compositionand patent are directed to a higher carbon,

quench hardening analysis.

There is no mention of precipitation hardening in the Finkl patent andthe relatively higher carbon composition of Pink] is not capable ofprecipitation hardening to any appreciable or useful degree.

- On the basis of the total austenite alloying effect and the totalferrite alloying effect, the ratio of these in the example cited byFinkl is 30.5 to 1.9 respectively or 16.1 which is quite remote from theteachings and objectives of the present inven tion.

While several illustrations of the present invention have beendescribed, it willbe understood that this invention may be otherwiseembodied within the scope of the following claims.

What is claimed is:

l. A precipitation hardening composition consisting essentially of about0.10 to 0.25 percent carbon; 1.00 percent maximum silicon: 1.00 percentmaximum manganese; 0.75 to 5.00 percent chromium; 1.00 to 4.00 percentnickel; 1.00 to 4.00 percent molybdenum; 1.25 to 3.50 percent copper;1.00 percent maximum aluminum; and 0.30 percent maximum nitrogen; andthe balance being iron; said precipitation hardening composition havinga ratio of total austenite alloying effect to total ferrite alloyingeffect of from about 1.00 to 1.65.

2. A precipitation hardened composition, consisting essentially of about0.10 to 0.20 percent carbon, 0.60 percent maximum silicon, 0.75 percentmaximum manganese, 1.25 to 3.00 percent chromium, 1.25 to 3.00 percentnickel, 1.50 to 3.00 percent molybdenum, and 1.50 to 3.00 percentcopper, with the remainder essentially iron; said composition beingcharacterized by an essentially uniform precipitation hardened structurein which precipitation of the age hardenable phase at the grainboundaries is substantially avoided.

3. A precipitation hardenable composition consisting essentially ofabout 0.10 to 0.20 percent carbon, 0.60 percent maximum silicon, 0.75percent maximum manganese, 1.25 to 3.00 percent chromium, 1.25 to 3.00percent nickel, 1.50 to 3.00 percent molybdenum, 1.50 to 3.00 percentcopper, and about 0.05 percent nitrogen with the balance beingsubstantially iron.

4. A low carbon alloy composition, precipitation hardenable to ahardness in excess of Rockwell C 50 on aging at about 1,050" F, saidalloy composition consisting essentially of about 0.10 to 0.25 percentcarbon, 1.00 percent maximum silicon, 1.00 percent maximum manganese,0.75 to 5.00 percent chromium, 1.00 to 4.00 percent nickel, 1.00 to 4.00percent molybdenum, and 1.25 to 3.50 percent copper, the balance ironwith incidental impurities; said alloy composition being characterizedby a total austenite alloying effect offrom about 8 to 16 and a totalferrite alloying effect of from about 7 to 14.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORREQTEN Patent No. 3,661,565 Dated May 9, 1972 Inventor(s) Richard F. Harvey It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

In the grant only, insert the attached pages, which are columns 5, 6, 7and 8.

Signed and sealed this 27th day of March 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR.

ROBERT GOTTSCHALK v Attestlng Officer Commissioner of Patents FORMO-1050 (10-69) USCOMM-DC 60376-P69 W LE5, GOVERNMENT PRINTING OFFICE:969 0'366-334.

2.00 PERCENT MOLYBDENUM, 2.65 PERCENT COPPER, AND THE BALANCESUBSTANTIALLY IRON WITH RESIDUAL ELEMENTS IN NORMAL AMOUNTS.
 2. Aprecipitation hardened composition, consisting essentially of about 0.10to 0.20 percent carbon, 0.60 percent maximum silicon, 0.75 percentmaximum manganese, 1.25 to 3.00 percent chromium, 1.25 to 3.00 percentnickel, 1.50 to 3.00 percent molybdenum, and 1.50 to 3.00 percentcopper, with the remainder essentially iron; said composition beingcharacterized by an essentially uniform precipitation hardened structurein which precipitation of the age hardenable phase at the grainboundaries is substantially avoided.
 3. A precipitation hardenablecomposition consisting essentially of about 0.10 to 0.20 percent carbon,0.60 percent maximum silicon, 0.75 percent maximum manganese, 1.25 to3.00 percent chromium, 1.25 to 3.00 percent nickel, 1.50 to 3.00 percentmolybdenum, 1.50 to 3.00 percent copper, and about 0.05 percent nitrogenwith the balance being substantially iron.
 4. A low carbon alloycomposition, precipitation hardenable to a hardness in excess ofRockwell C 50 on aging at about 1,050* F, said alloy compositionconsisting essentially of about 0.10 to 0.25 percent carbon, 1.00percent maximum silicon, 1.00 percent maximum manganese, 0.75 to 5.00percent chromium, 1.00 to 4.00 percent nickel, 1.00 to 4.00 percentmolybdenum, and 1.25 to 3.50 percent copper, the balance iron withincidental impurities; said alloy composition being characterized by atotal austenite alloying effect of from about 8 to 16 and a totalferrite alloying effect of from about 7 to 14.